import pandas as pd
df=pd.DataFrame({'商品':['苹果','香蕉','梨'],
                 '单价':[5,5,4],
                 '销量':[100,None,None],
                '库存':[50,None,70]})
print(df)
print(df.isnull())
print(df.notnull())
print(df.isnull().sum())
print(df.dropna())
print(df.dropna(axis=1))
print(df.dropna(thresh=3))
print(df.dropna(subset=['库存']))